This invention relates to ferritic alloys having good elevated temperature properties and more specifically to ferritic alloys having chromium and silicon with good oxidation resistance and creep strength up to 982.degree. C. (1800.degree. F.).
Low cost alloys having good strength and oxidation resistance at elevated temperatures have been sought for many years to replace stainless steels and nickel base alloys. The use of chromium, aluminum and silicon in ferrous base materials has been explored in many combinations as set forth below.
U.S. Pat. No. 3,698,964 (Caule et al.) discloses an alloy having up to 2% carbon, 1-5% chromium, 1-4% silicon, 1-4% aluminum and up to 2% copper. The preferred silicon alloy has 3% chromium, 2% silicon and 0.25% maximum carbon.
U.S. Pat. No. 3,782,925 (Brandis et al.) teaches 1-3.5% aluminum, 0.8-3% silicon and 10-15% chromium for oxidation resistance up to about 1,000.degree. C. (1832.degree. F.).
U.S. Pat. No. 3,905,780 (Jasper et al.) teaches a low alloy substrate for aluminum coating which has up to 0.13% carbon, 0.5-3% chromium, 0.8-3% aluminum, 0.4-1.5% silicon, 0.1-1% titanium and remainder substantially iron.
U.S. Pat. No. 4,261,739 (Douthett, et al.) has one family of alloys with 6% chromium, 0.01% carbon, 0.4-1% silicon, 1.5-2% aluminum, 0.4% titanium, 0.4% columbium and balance essentially iron. A final annealing temperature of 1010.degree.-1120.degree. C. (1850.degree.-2050.degree. F.) is critical in obtaining good creep strength in combination with uncombined columbium. An alloy having 4-7% chromium is stated to survive temperatures up to 815.degree. C. (1500.degree. F.).
U.S. Pat. No. 4,640,722 (Gorman) teaches a ferritic alloy having 0.05% maximum carbon, 1-2.25% silicon, 0.5% maximum aluminum, 8-20% chromium, 0.05% maximum nitrogen. The aluminum is restricted because of porosity problems in weld areas. Silicon is taught to have an adverse affect on creep strength unless a high temperature final anneal is given.
Austenitic nickel cast irons known as NI-RESIST (trademark of International Nickel Company) having up to 3% carbon, 1-5% silicon, up to 6% chromium, 13.5-36% nickel, up to 7.5% copper, 0.5-1.6% manganese, 0.12% maximum sulfur, 0.3% maximum phosphorus and balance iron have been used for some elevated temperature applications but are expensive due to the large amounts of nickel present.
The low chromium ferritic alloys in the past have relied mainly on aluminum to replace chromium for oxidation resistance except where weldability is important. Silicon, while known to improve oxidation resistance, has been used mainly in an amount below 2% and in combination with large amounts of aluminum. Silicon has been previously regarded to have an adverse influence on creep stength. Alloys having less than about 8% chromium have been difficult to maintain fully ferritic, particularly if the carbon and nitrogen levels are much above 0.03% each. The prior art alloys having large amounts of aluminum have suffered during the casting operation because of fluidity problems and poor slagging and oxide conditions. The cast product has not provided good as-cast toughness. Existing materials for high temperature applications are thus very expensive or provide less than the desired properties when balanced to be more economical.
It is an object of the present invention to provide a fully ferritic alloy which has good creep strength, oxidation resistance and casting properties at elevated temperatures. It is a further object to provide an alloy having higher silicon levels while still maintaining good creep strength. It is also an object of the present invention to improve the strength levels of the motlen alloy to provide improved casting properties. A still further object of the invention is to provide a low chromium alloy with higher levels of carbon and nitrogen and still maintain a fully ferritic structure including service at elevated temperatures. The ferritic alloy composition is balanced to provide elevated temperature properties equivalent or superior to the more expensive nickel cast irons and Type 409 stainless steel.